1. Field of the Invention
The present invention relates to hydraulic torque transmitting couplings and more particularly to control couplings of the rotational speed differential responsive type used as a driving torque distributing device, a differential slip limiting device, etc. in a multi-wheel drive vehicle such as a four-wheel drive vehicle.
2. Description of the Prior Art
An example of a prior art rotational speed differential responsive type control coupling is disclosed in Japanese Patent Provisional Publication No. 63-62635.
This control coupling includes concentric inner and outer rotary members, a fluid passage through which fluid flows in response to a relative rotation between the rotary members and an orifice disposed in the fluid passage for restricting fluid flow therethrough and thereby restricting relative rotation between the rotary members.
Since the orifice is not variable but fixed in opening irrespective of a vehicle running condition, the vehicle with such a control coupling cannot attain a desired driving stability and a desired steering characteristic under some particular running conditions, e.g., running with laterally opposed vehicle wheels being in engagement with road surfaces of largely different frictional resistances or running at an extremely low atmospheric temperature.
In order to solve the above problem, it has been proposed to utilize a variable orifice which is fully opened under the above described particular running conditions and partly closed to effect a restricted flow passage under a normal running condition. However, the orifice opening under a normal running condition is liable to vary due to a variation of hydraulic pressure to which it is subjected, resulting in a deteriorated control stability. Further, there has been design problems such as difficulties in attaining the fluid tightness of the variable orifice, determining the disposition of an actuator for the variable orifice, etc.